For me, the best game ever is Thief: The Dark Project. Then Thief: The Metal Age after that. Those two games sucked me in to them like black holes. I loved the whole sneaky stealth guy thing. Having the choice to use or not use weapons. Creeping up behind NPCs to pickpocket keys off of them. Knocking out guards with the blackjack. Then taking the unconscious bodies to a room and arranging them in compromising positions. I thought that was hilarious.